1. Field of the Invention
The present invention relates to the field of wavelength tunable semiconductor devices, and particularly to creating broadly tunable wavelength tunable semiconductor devices.
2. Description of Related Art
Wavelength-tunable lasers serve as a fundamental building block in constructing optical communication and sensing systems. Wavelength-tunable lasers play a central role in particular for dense wavelength division multiplexing (DWDM) systems that form the backbone of today""s optical communication network. The term xe2x80x9cwavelength-tunable laserxe2x80x9d is typically applied to a laser diode whose wavelength can be varied in a controlled manner while operating at a fixed heat sink temperature. At the 1550 nm wavelength regime on which most DWDM systems operate, a wavelength shift of 0.1 nm corresponds to a frequency shift of about 12.6 GHz. At a given heat sink temperature, the central wavelength of a conventional distributed feedback (DFB) laser diode may be red-shifted by as much as 0.3 nm or about 40 GHz due to the rise in the temperature of the junction by Ohmic losses. In contrast, at a given heat sink temperature, the wavelength of a tunable laser may vary by several nanometers, corresponding to hundreds or even thousands of GHz, covering several wavelength channels on the International Telecommunication Union (ITU) grid. Depending on the physical mechanisms of wavelength tuning, the lasing wavelength can be tuned in either positive (red) or negative (blue) direction. Controlled wavelength tunability offers many advantages over conventional fixed wavelength DFB lasers for DWDM operation. It enables advanced all-optical communication networks as opposed to today""s network where optics is mainly used for transmission and the network intelligence is performed in the electronic domain. All-optical networks can eliminate unnecessary E/O and O/E transitions and electronic speed bottlenecks to potentially achieve very significant performance and cost benefits. In addition, a less extensive inventory of wavelength-tunable lasers than of laser with a fixed wavelength is required. Keeping a large inventory of lasers for each and every wavelength channel can become a major cost issue. For advanced DWDM systems, the channel spacing can be as narrow as 50 GHz (or about 0.4 nm in wavelength, with as many as 200 optical channels occupying a wavelength range of about 80 nm. For the reasons stated above, wavelength-tunable lasers have attracted considerable interest in optoelectronic device research.
There exist different design principles for tunable lasers. Almost all wavelength-tunable laser designs make use of either the change of refractive indices of semiconductor or the change of laser cavity length to achieve wavelength tuning. For the former, common mechanisms for index change include thermal tuning, carrier density tuning (a combination of plasma effect, band-filling effect, and bandgap shrinkage effect), electro-optic tuning (linear or quadratic effect), and electrorefractive tuning (Franz-Keldysh or quantum confined Stark effect). For DFB lasers, the wavelength of the laser light propagating in the waveguide is basically determined by the grating period xcex9. The free-space lasing wavelength xcex is given by xcex=2 neff xcex9, where neff is the effective index of refraction of the waveguide and xcex9 is the period for first-order gratings. Accordingly, the change xcex94xcex in the lasing wavelength xcex is directly proportional to the change xcex9n of the index of refraction neff.
Accordingly, it is desirable to have structures and methods for generating broadly wavelength tunable semiconductor devices.
It is an object of the current invention to provide a method for decreasing the spontaneous recombination rate, B, in a DBR thereby increasing the tuning range of the DBR. According to the current invention, this may be achieved by creating electron confinement regions and hole confinement regions in the waveguide of the DBR.
A preferred process according to the current invention achieves this by engineering the band gaps of the DBR waveguide and cladding materials such that an electron confinement region and a hole confinement region are created in the waveguide of the DBR, thereby reducing the recombination rate and increasing the tuning range. Preferably, the materials selected for use in the DBR may be lattice matched.
Optionally, alternate methods according to the current invention may create two or more thin electron confinement regions and/or two or more thin hole confinement regions. In some cases, multiple thin confinement regions may be used to take advantage of strain compensation in thinner layers thereby broadening the choices of materials appropriate for use in the broadly tunable DBR.
Optionally, a variety of DBR designs comprising graded materials and/or graded interfaces may be used to provide effective electron and/or hole confinement regions may be created using alternate methods according to the current invention.
Optionally, a variety of optical devices such as lasers and/or integrated opto-electronic devices may be created using an alternate method according to the current invention by incorporating one or more conventional processing steps.
Advantageously, the current invention enables the production of DBRs and lasers with broad wavelength tuning ranges making the use of tunable wavelength devices more attractive for a variety of optical networking applications.